There is known a vehicle wherein a clutch provided in a power transmitting path between an engine and drive wheels is released for holding the engine in an operated state during stopping of the vehicle, and is engaged to start the vehicle. Publications identified below show examples of such a vehicle. In a vehicle disclosed in JP-1-285427A, for example, there is provided an actuator for engaging and releasing the clutch. This actuator permits a control of a partially engaged state of the clutch according to an operating amount of an accelerator pedal upon starting of the vehicle, thereby making it possible to suitably start the vehicle while assuring an intricate control of the vehicle running speed.    JP-1-285427 A    JP-5-215216 A    JP-2004-28056 A    JP-1-240328 A    JP-2003-130202 A    JP-2003-301731 A
On the other hand, there is also known a vehicle which includes a differential mechanism for distributing an output of an engine to a first electric motor and an output shaft, and a second electric motor provided in a power transmitting path between the output shaft of the differential mechanism and vehicle drive wheels, and which can be suitably started, and wherein the engine can be held in an operated state upon stopping or low-speed running of the vehicle, even in the absence of a mechanism (device) such as the above-described clutch or a fluid-operated power transmitting device in the above-indicated power transmitting path, which mechanism includes input and output rotary members that are rotatable relative to each other. JP-2003-130202A and JP-2003-301731A identified above disclose drive systems for hybrid vehicles as an example of such a vehicle. In these hybrid vehicle drive systems, the differential mechanism is constituted by a planetary gear set operable to perform a differential function permitting the engine to be held in the operated state upon stopping of the vehicle, and is operable as a transmission, for example, as an electrically controlled continuously variable transmission that is controlled by a control device, to mechanically transmit a major portion of a drive force of the engine to the drive wheels, and to electrically transmit the remaining portion of the drive force through an electric path from the first electric motor to the second electric motor, so that the speed ratio of the transmission is electrically variable, whereby the vehicle can be run with improved fuel economy while the engine is kept in an optimum state.
In the hybrid vehicle drive system as disclosed in the above-identified publication JP-2003-130202A wherein the differential mechanism is operated as the transmission the speed ratio of which is electrically changed, however, the first electric motor which generates a reaction torque corresponding to an output torque of the engine (hereinafter referred to “engine torque”) is required to be large-sized with an increase in the required output capacity of the engine, since the reaction torque that must be generated by the first electric motor increases with an increase in the engine torque required to assure desired acceleration of the vehicle upon starting of the vehicle.
It is also noted that the differential mechanism provided in the hybrid vehicle drive system of the publication indicated just above is limited in its torque capacity due to its design, so that the differential mechanism is required to be large-sized with an increase in the required output capacity of the engine. This increase of the differential mechanism is not desired for the vehicle. Where the differential mechanism is arranged to distribute the output of the engine to the first electric motor and a power transmitting member, for instance, the differential mechanism is controlled so as to function a transmission the speed ratio of which is electrically controlled, the first electric motor generates a reaction torque corresponding to an output torque of the engine (hereinafter referred to as “engine torque”) and is therefore required to be large-sized with an increase in the required output capacity of the engine, since the reaction torque that must be generated by the first electric motor increases with an increase in the engine torque required to assure desired acceleration of the vehicle upon starting of the vehicle.
It is possible to temporarily restrict the engine torque or limit the maximum engine torque to a value not exceeding the maximum reaction torque that can be generated by the first electric motor, for protecting the first electric motor, without increasing the size of the differential mechanism or first electric motor. In this case, the torque to be transmitted to the vehicle drive wheels is reduced, resulting in a possibility of deterioration of the vehicle acceleration or a risk of a failure to obtain the desired vehicle acceleration characteristics.
The vehicular drive system described above is further arranged to drive select one or both of the electric motor and the engine as the vehicle drive power source, depending upon the desired condition of the vehicle such as stopping, low-load running or acceleration of the vehicle, so that the engine is started and stopped, according to the desired condition of the vehicle. For example, the engine is started to select the engine as the vehicle drive power source, if the required vehicle drive torque is increased to obtain the desired vehicle acceleration during running of the vehicle with the electric motor selected as the vehicle drive power source. In this case, the engine torque is not generated for at least a length of time required to start the engine. Namely, there is a possibility that the generation of the engine torque is delayed in the case of starting of the engine, contrary to the case where the engine is held in the operated state. As a result, the transmission of the drive torque to the drive wheels is accordingly delayed, leading to deterioration of the vehicle acceleration or a risk of a failure to obtain the desired vehicle acceleration characteristics.
The present invention was made in view of the background art descried above. It is a first object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to perform a differential function for distributing an output of an engine to a first electric motor and an output shaft, and an electric motor provided in a power transmitting path between the differential mechanism and a vehicle drive wheel, which control apparatus prevents an increase in the required size of the first electric motor which generates a reaction torque corresponding to a torque of the engine. It is a second object of the invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to transmit the output of the engine to a power transmitting member, and an electric motor operable to transmit its output to the vehicle drive wheel, which control apparatus reduces deterioration of the vehicle acceleration characteristics.